<!DOCTYPE html>
<html>
 <head>
  <title>JS事件-计算器</title>  
  <script type="text/javascript">
   function count(){
       
    //获取第一个输入框的值
    var text1 = document.getElementById("txt1").value;
  //获取第二个输入框的值
   var text2 = document.getElementById("txt2").value;
  //获取选择框的值
   var symbol = document.getElementById("select").value;
  //获取通过下拉框来选择的值来改变加减乘除的运算法则
    //设置结果输入框的值 
    var result;
    switch(symbol){
      case "+":
      result=(parseInt(text1)+parseInt(text2));
      break;
      case "-":
      result= (parseInt(text1)-parseInt(text2));
      break;
      case "*":
      result= (parseInt(text1)*parseInt(text2));
      break;
      case "/":
      result= (parseInt(text1)/parseInt(text2));
      break;
    }
    document.getElementById("fruit").value = result;
   }
  </script> 
 </head> 
 <body>
   <input type='text' id='txt1' /> 
   <select id='select'>
    <option value='+'>+</option>
    <option value="-">-</option>
    <option value="*">*</option>
    <option value="/">/</option>
   </select>
   <input type='text' id='txt2' /> 
   <input type='button' value=' = ' onclick="count()"/> <!--通过 = 按钮来调用创建的函数，得到结果--> 
   <input type='text' id='fruit' />   
 </body>
</html>